High dielectric constant (high-k) materials are desirable for use as capacitor dielectrics and for use as gate dielectrics in future generations of electronic devices. The first high-k materials used as capacitor dielectrics were tantalum oxide and aluminum oxide materials. Currently, hafnium aluminum oxide materials are being implemented as capacitor dielectrics in DRAM production. Similarly, hafnium-based dielectrics are expected to enter production as gate dielectrics, thereby replacing or complimenting the current silicon-containing materials.
The most common methods of depositing high-k dielectrics include physical vapor deposition (PVD), chemical vapor deposition (CVD) and atomic layer deposition (ALD). The advantages of using ALD over PVD and CVD methods include improved thickness control for thin films, improved uniformity across the wafer and improved conformality over high aspect ratio structures.
The atomic layer deposition process includes separate pulses of reactive vapor streams to a process chamber containing a substrate, where the pulses can be separated by either purging or evacuating. During each pulse, a self-limited chemisorbed layer is formed on the surface of the wafer, which layer then reacts with the component included in the next pulse. Purging or evacuation between each pulse is used to reduce or eliminate gas phase mixing of the reactive vapor streams. The typical ALD process results in well-controlled sub-monolayer or near monolayer growth per cycle.
One representative case of ALD is deposition of aluminum oxide (Al2O3) from trimethylaluminum and water. In this ALD process, a pulse of trimethylaluminum will react with hydroxyl groups on the surface of a heated substrate to form a chemisorbed layer of methyl-aluminum moieties that are self-limited to less than a monolayer. The reaction chamber is then purged or evacuated to remove unreacted trimethylaluminum as well as any vapor phase reaction by-products. A pulse of water vapor is then introduced which reacts with the surface aluminum-methyl bonds and regenerates a hydroxylated surface. By repeating the above deposition cycle it is possible to realize layer by layer film growth of about 1 angstrom (10−10 m) per cycle. By selecting different reactive precursors and gases, it is possible to deposit many different types of films using ALD processes.
Current high-k dielectric materials under evaluation suffer from various problems. Some of the problems encountered include film crystallization during anneals, growth of interfacial layers during deposition and further processing, large densities of interface traps, reduced channel mobility, reaction with poly-silicon gates, Fermi level pinning with metal gates, and leakage current through the films in the semiconductor device. Other problems encountered with current high-k dielectric materials include dielectric constants that are too low compared to desired values for advanced semiconductor devices.